Agradecimientos:
We acknowledge financial support of the Direccion General de Investigacion Cientifica y Tecnica (Spain) through two projects: PB92-0248 (F.C.M., J.M.M., and A.S.) and PB91-0378 (J.A.C.).

Resumen:

We introduce two simple two-dimensional lattice models to study traffic flow in cities. We have
found that a few basic elements give rise to the characteristic phase diagram of a first-order phase
transition from a freely moving phase to a jammed state, withWe introduce two simple two-dimensional lattice models to study traffic flow in cities. We have
found that a few basic elements give rise to the characteristic phase diagram of a first-order phase
transition from a freely moving phase to a jammed state, with a critical point. The jammed phase
presents new transitions corresponding to structural transformations of the jam. We discuss their
relevance in the infinite-size limit.[+][-]